The invention relates to a method of fabricating an epitaxial substrate which is suitable for use in fabricating good quality homo- or hetero-epitaxial films or layers.
Epitaxy is a process during which a crystalline layer of a material is deposited on a substrate, which is also crystalline. Epitaxial growth is characterized by having the crystalline structure of the substrate reproduced in the epitaxial layer of the material that is grown. Consequently, the defects present in the substrate are usually reproduced in the epitaxial layer. Epitaxial layers are typically used in electronic or optoelectronic applications. Of particular interest are, for example, gallium nitride epitaxial layers which, due to their large band gap, are used in blue, violet or ultraviolet laser diodes.
Epitaxial techniques can be essentially grouped into two families. First, there is homoepitaxy, wherein the material to be grown is of the same nature as the substrate. This means that the crystallographic structure and the chemical nature of the substrate and the resulting layer are essentially identical. Typical examples used in industry are the homoepitaxy of silicon on a silicon substrate, or the epitaxial growth of gallium arsenide on a substrate of gallium arsenide.
Of even greater interest is heteroepitaxy, wherein a film layer is grown on a substrate of a different nature. This is especially important where the desired material is not available in the form of a crystalline substrate. There are two major problems inherent with heteroepitaxy: the difference in the crystalline structure of the two materials, and the difference in their thermal expansion coefficients. These differences lead to stress inside the film and consequently to defects such as dislocations.
In addition to the above-mentioned problems that result in insufficient epitaxial layer quality, it is also possible that the substrate, due to its intrinsic characteristics, is not suitable for a particular device application that would use the special characteristics of the epitaxial layer. For example, when growing gallium nitride on silicon carbide relatively good epitaxial growth could be achieved. However, if the gallium nitride structure is intended for fabrication of a light-emitting device, the silicon carbide substrate would not be advantageous, as it would trap too much light.
Several approaches are known for overcoming the above-mentioned problems. However, none proposes a substrate which allows homo- as well as heteroepitaxial Growth and overcomes each of the three distinct problems mentioned above. For example U.S. Pat. No. 5,759,898 discloses that it is possible to successfully grow an epitaxial silicon germanium thin film on a silicon-on-insulator wafer. In this case, the silicon germanium layer grows on a thin silicon film (about 16 nm), which itself is positioned on a layer of silicon dioxide, which is in turn positioned on a silicon wafer. In such a structure, the appearance of dislocations in the silicon germanium film is greatly diminished in comparison to a silicon germanium film directly grown on a bulk silicon substrate.
Another approach is described in International Publication WO 99/39377, wherein a “compliant” substrate is created by ion implantation. In this approach, ion implantation is used to obtain a weakened layer inside the material, which also creates a very thin layer on the top. This top layer is partially isolated from the substrate via the weakened layer, and to some extent absorbs the crystalline structure and thermal expansion mismatch between the top layer of the original substrate and the epitaxial layer to be grown, thereby causing at least a partial relaxation of the stress of the epitaxial layer.
These two approaches overcome to some extent the above-mentioned problems. However, the presence of the original substrate imposes a negative influence that is not excluded. In some cases the presence of the original substrate is incompatible with the desired application of the epitaxial layer that is grown, so that further processing steps are needed to remove the original substrate, which leads to higher production costs. Thus, improvements in these products and the processes of making them are desired.